The invention relates to a cathode-ray tube comprising, in an envelope, a target and an electron gun centered along an axis and serving to generate an electron beam directed on the target. The electron gun comprises a cathode and an anode, the anode having a small aperture to limit the electron beam. The cathode-ray tube further comprises a focusing lens to focus the electron beam onto the target. In the focusing lens, the anode, constitutes the first electrode in the direction in which the electrons travel.
Such a cathode-ray tube which is used for recording television pictures is generally known as a vidicon. Between the cathode and the target of the vidicon are present one or more diaphragms to limit the electron beam. The tube furthermore comprises a focusing lens to focus the electron beam on the target.
Such a cathode-ray tube is known from the article "Een Experimentele Kleine Kleurentelevisie-camera" in Phillips Technisch Tijdschrift ("An Experimental Light-Weight Colour Television Camera" in Phillips Technical Review), Volume 29, 1968, No. 11, pages 325-335 in which a television camera tube of the PLUMBICON (trademark) brand is described.
In a PLUMBICON (trademark) brand vidicon, the target consists of a photoconductive layer, comprising a major amount of lead monoxide, which is provided on a transparent signal plate. The free surface of the photoconductive layer faces the electron gun.
The operation of a PLUMBICON (trademark) brand vidicon is as follows. The signal plate is connected to a voltage source via a signal resistor. The potential of the signal plate is positive relative to the cathode potential which is, for reference purposes, zero volts. The scene to be recorded is projected through the transparent signal layer on the photoconductive layer. Under the influence of the positive potential of the signal plate the potentials of local subareas of the target increase as a result of photoconduction. As a result of this a potential image is formed on the free surface of the target, the potentials of the local subareas being dependent on the incident light intensity. The potential image on the target is scanned by an electron beam according to a raster of substantially parallel lines. The potentials of the surface elements of the target are periodically reduced to the potential of the cathode by the scanning electron beam. As a result, an output signal which is proportional image appears as voltage to the original potential fluctuations across the signal resistor.
In the television camera tube described in the above-cited article, the electron gun is formed by a cathode, a grid and an anode. As a result of the lens action between the electrodes, the electron beam is focused between the cathode and the anode in a so-called crossover. The crossover is focused on the target by a focusing lens. The focusing lens comprises three cylindrical electrodes of which the first electrode is formed by the anode. In order to obtain the desired beam at the area of the focusing lens, a beam-limiting diaphragm is provided in the cylindrical anode.
In the above-cited article the cylindrical anode is at a potential of 300 V relative to the cathode potential. At such a high voltage, positive ions are easily formed at the area of the crossover and at the area of the focusing lens as a result of collisions of beam electrons with residual gas in the tube. As a result of the diaphragm in the cylindrical anode the beam current at the area of the focusing lens is much smaller than at the area of the crossover. The number of positive ions formed at the focusing lens thus is much smaller than the number of positive ions formed at the crossover.
The positive ions formed in the manner discussed above follow substantially the original electron path in the opposite direction. Since the electron beam at the area of the focusing lens has a very small apex angle and thus extends substantially parallel to the axis of the electron gun, substantially all the positive ions formed at the focusing lens pass back through the diaphragm. Thus substantially all the positive ions formed at the crossover and at the focusing lens reach the cathode surface; and the ions reach the cathode surface in those places where the cathode emits electrons. In addition a certain focusing of the positive ions on the cathode occurs. The result of this is that the emission of the cathode rapidly deteriorates so that the life of the tube is restricted.